The present invention provides a device for growing microorganisms configured to spread an aqueous sample to cover a designated culture surface. The end user is able to obtain a uniformly spread sample without any additional equipment or manipulation of the sample.
Media for culturing microorganisms are generally prepared by dispersing a solidifying agent in an aqueous solution containing nutrients and other ingredients necessary for the growth of specific microorganisms. Conventional solidifying agents, such as agar, are often inconvenient for the end user. Agar medium is typically prepared in bulk, sterilized, then melted in boiling water or by exposure to flowing steam. The hot agar must be carefully cooled to approximately 45xc2x0 C. before it can be poured into petri dishes. The cooled, but still liquefied, medium is aliquoted, poured into the petri dish containing the microbiological sample, mixed with the sample and allowed to solidify. After the agar hardens, the plates are incubated at a prescribed temperature for a prescribed period of time. After incubation, the number and variety of colonies growing in each dish is counted by visual inspection. In this way, one can determine the number and variety of microorganisms or colony-forming units present in an aqueous sample.
A dry culture medium device is disclosed in U.S. Pat. No. 4,565,783, entitled xe2x80x9cDry Culture Media,xe2x80x9d granted to Hansen (the ""783 patent). The device of the ""783 patent comprises a bottom member with an adhesive coating and a further coating of cold-water-soluble powder adhered uniformly to the adhesive coating. The powder comprises one or more ingredients such as a gelling agent, one or more nutrients, or a mixture thereof. A preferred embodiment further comprises a cover sheet releasably adhered to at least a portion of the bottom member.
A shortcoming of the device disclosed in the ""783 patent is that there is no means by which one can control the dispersion of an aqueous sample deposited on the bottom member. If one simply allows the aqueous sample to spread on its own, the result may be an inoculum of irregular shape and dimension. Such irregularity can lead to less-than-optimum concentrations of culture medium components, e.g., gelling agents, nutrients, inhibitors, indicators, and the like. Also, one risks having the aqueous sample spread beyond the boundary of the bottom member, thereby spilling culture liquid, creating mess and potentially contaminating the work area. Alternatively, one may employ a sample-restraining device to restrain spreading of the sample. An example of a device suitable for this purpose comprises a block with a cavity exposed on one surface. An aqueous sample is applied to the bottom member of the device disclosed in the ""783 patent, the cover sheet is lowered over the sample and the sample-restraining block is placed on the cover sheet with the cavity side down. The walls of the cavity define the extent to which the sample is allowed to spread. The sample-restraining block is removed after the gelling agent of the device begins to gel. This process is cumbersome because when culturing a large number of samples, one must either have a large supply of sample-restraining blocks or wait for each sample to begin to gel before proceeding to the next culture.
The ""783 patent also discloses an embodiment of the device that employs a spacer attached to the upper surface of the substrate. The spacer has a hole cut in its center that forms a well of predetermined size, shape and volume that is designed to confine the culture medium following hydration. However, there may be disadvantages in utilizing a device having such a sample well. For example, the sample volume may be inadequate to completely fill the well so that there is an undesirable air space between the surface of the liquid sample and the lowered cover sheet. Or, alternatively, the sample volume may be so great that the sample overflows the well upon lowering the cover sheet upon the liquid sample surface. If the sample reaches the top of the well, it will flow over the surface of the spacer through the narrow gap between the top surface of the spacer and the top sheet.
U.S. Pat. No. 6,022,682, entitled, xe2x80x9cArticle and Method for Detection of Enterotoxigenic Staphylococci,xe2x80x9d granted to Mach et al. (the ""682 patent) teaches the use of a disc-shaped article for detecting or confirming the presence of thermostable nuclease positive, potentially enterotoxigenic staphylococci in a sample. The article contains a chemical composition specific for the detection of specific strains of staphylococci. The article is particularly adapted for detecting the presence of enterotoxigenic staphylococci such as S. aureus in a pre-grown, gel-based bacterial culture. However, the article of the ""682 patent is not an integral part of or attached to a dry culture device, is not designed to receive a liquid aqueous sample, and is removable from the dry culture device during use.
What is needed is a device for growing microorganisms that, without additional labor or equipment required of the end user, a) limits the spread of a liquid sample so that the sample does not spill, and b) results in a spread sample that is uniformly shaped and sized.
The present invention provides a device for growing microorganisms that facilitates spreading of an aqueous sample uniformly, to substantial homogeneity, and with reduced risk of spilling, with no additional effort or manipulation by the end user.
In several places throughout the specification, guidance is provided through lists of examples. In each instance, the recited list serves only as a representative group and should not be interpreted as an exclusive list.
The present invention provides a device for growing microorganisms comprising a substrate having a top side, a pedestal mounted on the top side of the substrate and comprising a non-absorbent culture surface, a flexible cover sheet attached to the substrate and comprising a bottom surface, and a culture medium deposited on one or more surfaces selected from the culture surface and the cover sheet bottom surface.
In another aspect, the present invention also provides a method of growing microorganisms in which the end user is not required to manipulate the aqueous sample after plating in order to obtain a spread sample area of substantially uniform size and shape.
The present invention provides a method of culturing microorganisms comprising providing a device for growing microorganisms comprising a substrate having a top side, a pedestal mounted on the top side of the substrate and comprising a non-absorbent culture surface, a flexible cover sheet attached to the substrate and comprising a bottom surface, and a culture medium deposited on one or more surfaces selected from the culture surface and the cover sheet bottom surface; placing a sample on the culture surface; covering the sample with the cover sheet; and incubating the device.
In yet another aspect, the present invention provides a method of detecting microorganisms, wherein the method of culturing microorganisms described above further comprises detecting the microorganisms.